Reorganize repo: backend/, deploy/, docs/ layout + AGENTS.md

Restructure the monorepo into clear top-level directories:
- backend/: Go module root (cmd, internal, pkg, configs, migrations,
  docs/swagger, scripts, tests, go.mod, Makefile, .air.toml)
- deploy/: Docker (Dockerfile, docker-compose*), quickstart, fluentd
- docs/: project documentation + reports/ (moved from repo root)
- AGENTS.md: new AI coding-agent guide at repo root

Update all references to the new layout:
- Dockerfile: COPY backend/go.mod, COPY backend/ (context = repo root)
- docker-compose files: context ../.., dockerfile deploy/docker/Dockerfile,
  env_file ../../.env, volume mounts ../../backend:/app
- deploy/quickstart/compose.yaml: dockerfile deploy/docker/Dockerfile
- CI: working-directory: backend for go commands, file deploy/docker/Dockerfile,
  coverage path backend/coverage.out, health_check backend/scripts/
- backend/Makefile: docker target uses -f ../deploy/docker/Dockerfile ../
- README: architecture tree, quickstart, config paths updated

Move root stray scripts (rename_models.*, run_m11_tests.sh, verify_build.sh,
gorm_bool_main.go) to backend/scripts/legacy/. All moves via git mv to
preserve history. Build, vet, SQLite tests, and docker compose config verified.

backend/internal/handler/api/v1/delivery_status_handler.go

@@ -0,0 +1,52 @@
+package v1
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+
+	"github.com/gochat/gochat/internal/service"
+	"github.com/gochat/gochat/pkg/response"
+)
+
+// DeliveryStatusHandler handles message delivery status endpoints.
+// Reference: Chatwoot message delivery status per recipient tracking
+// GET /api/v1/accounts/:account_id/conversations/:conversation_id/messages/:message_id/delivery_status
+type DeliveryStatusHandler struct {
+	svc *service.DeliveryStatusService
+}
+
+// NewDeliveryStatusHandler creates a new DeliveryStatusHandler.
+func NewDeliveryStatusHandler(svc *service.DeliveryStatusService) *DeliveryStatusHandler {
+	return &DeliveryStatusHandler{svc: svc}
+}
+
+// List retrieves all delivery statuses for a given message.
+// GET /api/v1/accounts/:account_id/conversations/:conversation_id/messages/:message_id/delivery_status
+func (h *DeliveryStatusHandler) List(c *gin.Context) {
+	accountID, err := parseUintParam(c, "account_id")
+	if err != nil {
+		response.AbortWithStatusError(c, http.StatusBadRequest, response.ErrBadRequest, "invalid account_id")
+		return
+	}
+
+	conversationID, err := parseUintParam(c, "conversation_id")
+	if err != nil {
+		response.AbortWithStatusError(c, http.StatusBadRequest, response.ErrBadRequest, "invalid conversation_id")
+		return
+	}
+
+	messageID, err := parseUintParam(c, "message_id")
+	if err != nil {
+		response.AbortWithStatusError(c, http.StatusBadRequest, response.ErrBadRequest, "invalid message_id")
+		return
+	}
+
+	statuses, svcErr := h.svc.ListByMessage(c.Request.Context(), accountID, conversationID, messageID)
+	if svcErr != nil {
+		handleServiceError(c, svcErr)
+		return
+	}
+
+	response.OK(c, statuses)
+}